1. Field of the Invention
The present invention concerns the temporary interruption of a call between first and second terminals in order to transmit data to the first terminal via an equipment such as a switching center through which the call between the first and second terminals passes, before re-establishing the call. The data to be transmitted comprises, for example, characters identifying a third terminal calling the first terminal to which the data is addressed.
The present invention is more particularly concerned with a temporary interruption of a call in an analog telephone network.
2. Description of the Prior Art
In a known method, the switching center with serves the first terminal, temporarily interrupts the call between the first and second telephone terminals if the switching center receives a call request signal from the third terminal. The switching center then transmits to the first telephone terminal a user alerting signal such as a signal audible to the user of the first telephone terminal.
The switching center then transmits a terminal alerting signal to the first telephone terminal. The latter, if it is designed to process the terminal alerting signal, responds with an acknowledgment signal. After receiving the acknowledgment signal, the switching center transmits a data message to the telephone terminal concerned and then re-establishes the momentarily interrupted call between the first and second telephone terminals. If the switching center does not receive a acknowledgment signal it re-establishes the call between the first and second telephone terminals.
This known method, which is described in more detail hereinafter with reference to FIG. 2, applies only to the interruption of telephone calls. Telematics calls, or mixed calls successively transmitting speech and data, are disturbed by temporary interruption caused by the known method.
These disturbances are due notably to erroneous recognition of the terminal alerting signal by the first terminal during the call itself, or to failure to detect the terminal alerting signal because it has been disturbed by signals transmitted by the first terminal. These disturbances are also due to the fact that the first terminal or second terminal cannot accept an interruption of sufficient duration.
The consequences of the disturbance include particularly irreversible clearings down of telematics or mixed calls and non-detections and erroneous detections of the temporary interruption.
The switching center cannot distinguish between different types of call to be interrupted and must be able to interrupt any type of call using a single method.